But if the defence depends upon a river, or other de file, a central post is generally chosen and occupied, from which the passage may always be opposed ; ur, if there be no such post, the movements of the enemy must be accurately observed and followed, from the first approach of the quarters, until the entire junction of the army ; or separate corps are drawn together in front of the defile, which must always be able to unite sooner than the enemy, or at least to maintain their ground until they receive assistance ; and which must be occasionally reinforced, in proportion to the means employed by the enemy.
The same rule may be applied to the drawing toge ther of an army at the opening of a new campaign. It must be observed, that the attacking party is generally the first to collect his army : while the party acting on the defensive must almost always wait to see the ene my's object before he can do any thing. On some oc casions, however, the latter party may be the first to collect his army ; if, for example, he wishes to consume the provisions in a district before the enemy comes in to the field, in order to frustrate the enterprise he may have in view ; or when he is going to perform some work necessary for his defence, which must be covered by the army. In drawing an army together, attention most be paid to the season of the year, and also to the comfort of the troops, who ought not to be exposed to great hardships, until they have become habituated to active service.
Modern writers on the art of war have denominated the chain of magazines established at the commence ment of a campaign, the Basis of military operations ; and the roads by which an arm) receives its supplies from the magazines, are called Lines of operation. The situation of the principal magazine, and the length and direction of the lines of operation, are considered of the highest importance.
Single lines of operation are preferable to double ones, especially when the latter have an eccentric di rection, which exposes the different parts of an army to be attacked and beaten in detail. It is always of advantage, therefore, to give to our own operations an interior direction, and, if possible, to induce the enemy to adopt exterior lines of operation. If it be neces sary to form two interior lines, in order to oppose two lines of operation of the enemy, we ought not to sepa rate them too far ; because the army acting upon that which is weakest, might be attacked and beaten, with such effect as no advantage obtained on the other line could compensate. For a simd r re ason, a double line
ought not to be presented against a single line, nor an extended line against one that is concentrated. In forming lines of operation, the chief object to he at tended to is, the facility of conjunction and simulta neous co-operation. Lines of operation ought to be direct,—consequently such as will enable our tioups to march with greater rapidity towards any point that is in danger, than those of the enemy, and thus to present a mass to isolated parts.
A twofold line of operations may be considered good, when the enemy shall likewise have formed a double line ; provided the parts of his line have an ex terior direction, while y ours have an interior one ; and his divisions being at greater distances than your ow n, cannot unite v. idiom being pees iously attacked by yours in mass. An army possessing lines of ope ration more contracted than those of the enemy can, by strategical movements, overpower the enemy's di visions successively, by alternately collecting the mass of its forces, and attacking them one alter another. In order to insure the success of these movements, it will be necessary to leave a small division to ourise that of the enemy, which it is finttnded to keep in check ; with orders not to engage, but to use every attto ar rest or suspend its advance, by defending the dillies, heights, and risers ; and lastly, to fall back towards the army.* Iichce it follows, that a twofold line of operations, embracing the extremities of a more con tracted line, will inevitably be ruined, if the army, acting on the shorter space, know how to profit by the advantage of its situation, and the rapidity with which it can act within its own line. A twofold line of ope rations, opposed to a single one, will be c xposed to still greater danger, if its parts are several day s' march asunder, because the difficulty will then be much greater of uniting to resist any concentrated eff.•t of the enemy. All interior and single lines of operations, therefore, will be the most secure. They offer no ad vantage to the enemy, but, on the contrary, should he have the imprudence a contrary system, are calculated to bring their whole mass into action against his isolated dis isions.